In an aerial tanker airplane used by the military known as the Boeing KC-135, a flying refueling boom is supported from the airplane about a fixed vertical axis for free pivotal movement in a sideward direction or in azimuth; and the boom is also supported for free pivotal movement about a lateral axis for up and down movement or in elevation. The means for moving the boom about these axes, comprises a pair of aerodynamic surfaces formed into a Vee and known as ruddevators. When the ruddevators are moved collectively to a negative angle-of-attack, the boom will be moved downwardly; and a differential change in the angle-of-attack of the ruddevators will move the boom sidewardly.
Included in the ruddevator control system is a pantographing cable system, similar to that of a drafting machine, which functions to automatically position the ruddevators when the boom is moved by other than the boom operator's control stick; e.g., when engaged with a receiver airplane, and the receiver airplane moves the boom in elevation, the ruddevators will automatically pantograph collectively. Also, when the receiver airplane pulls the boom off to one side, the ruddevators will pantograph differentially. This aids in alleviating the additional air loads that would be imposed on the boom and ruddevators as a result of the boom being displaced without any control input from the boom operator, if the pantographing system were not utilized.
The present aerial tankers like the Boeing 747, the Boeing 707, and the Boeing KC-135, as well as the previous tankers like the Boeing KC-29 and KC-97, have used a flying boom type of aerial refueling with considerable success. However, limitations in the aerodynamic performance of that known boom become apparent when it is used at the ever increasing speeds and altitudes required to refuel the modern high speed military aircraft. In order to provide a flying boom system of aerial refueling that would permit fuel transfer between the tanker airplane and the receiver airplane, at the air speeds and altitudes desirable for receiver airplanes, there must be adequate aerodynamic control forces available from the airfoil surfaces activating the boom, in order to position it throughout a spaced envelope large enough for the receiver airplane to remain within, with a reasonable effort on the part of the pilot of the receiver airplane and the boom operator.
The limiting factor in the refueling envelope size is the boom disconnect envelope, i.e. sufficient aerodynamic ruddevator control force must exist to permit a safe extraction of the boom nozzle from the receiver receptacle beyond the normal limits of the refueling envelope. For the present known tanker airplanes like the Boeing 747, the Boeing 707, and the Boeing KC-135, the increase in airspeed or Mach number of the tanker airplane to that desired for optimum refueling of the future inventory receiver airplanes, could reduce the boom maneuvering envelope to the point where the refueling operation would be compromised; e.g., if due to restrictions on the operation of the boom, the refueling operation requires that it be done at lower airspeeds, a considerable loss in altitude as well as true airspeed would be necessary for the receiver airplane. The range of the receiver aircraft would be degraded since additional fuel is consumed in returning to cruise airspeed and altitude after refueling. This also results in increased vulnerability of both aircraft to enemy interception as well as the more severe weather conditions at the lower altitudes which could complicate the rendezvous and refueling hook-up operation.
One of the inherent aspects of the present known KC-135 boom ruddevator control system, is that when the boom is moved all the way over to one side of the azimuth envelope, there is quite a bit of air drag generated by the boom; and this results in an increase in the aerodynamic control force requirements of the ruddevators in maintaining that extreme azimuth position. Also, at this extremity of the azimuth envelope, the wake flow from the boom partially blanks out the air flow over one of the ruddevators, reducing the maximum control force available. It could be said, that with the KC-135 boom having a fixed vertical and horizontal hinge geometry, that the boom yaws right out of control power, i.e., it can not be flown nor driven by the ruddevators to the extremities of the desired envelope.
A more detailed explanation of the manner in which this control system operates is disclosed in U.S. Pat. No. 2,960,295 to Schulze.
Aerodynamic yaw drag increases the lateral aerodynamic control power required from the generally known ruddevator surfaces. However, if the aerodynamic yaw drag of the boom could be reduced at certain lateral linear displacements, the lateral displacement capability of the boom could be increased to provide adequate boom movement and improve the efficiency of aerial refueling.
Previously known efforts have pursued the increase in the aerodynamic lift of the ruddevator surfaces as a method for increasing the control power performance of the refueling boom. However, this approach was found not to be cost effective on the present boom due to the major structural design changes necessary to carry the increased loads. Additional efforts have investigated a change in the pivot axis system which included setting the pivot axis, about which the boom moves laterally, at a fixed angle of inclination other than near vertical. This resulted in a combination of a rolling and yawing action of the boom for a given sideward or lateral displacement angle of the boom, and reduced the aerodynamic yaw drag. Further, by carrying the inclination of this boom lateral motion pivot axis to the extreme, and having it at an essentially horizontal position, almost no lateral or sideward movement of the boom against the freestream occurs. The motion is mostly a rolling action of the boom about its centerline. This type of boom geometry has stability problems and would require considerable development effort before it would be operationally feasible. In addition, the lateral displacement capability of this type boom is much greater at the lower portions of the refueling envelope than at the upper portions. Safety margins are thus compromised in the upper portions of the envelope.
Safety considerations dictate that a continuous aerodynamic control force capability be provided throughout the normal refueling envelope; and that this aerodynamic control force be of sufficient magnitude that the boom can be maneuvered clear of an over-running receiver airplane. Sufficient vertical manueverability must exist to fly the boom at least to the horizon to evade receiver aircraft.
The lateral movement of the boom is equally important to its vertical movement when evading an over-running receiver airplane. Boom operators generally agree, that the necessity for maximum lateral displacement of the boom is greatest when the boom is at the inner upper and inner lower corners of the refueling disconnect envelope; because, the over-runs of the receiver airplane with the boom at the inner lower corner of the refueling envelope, have resulted in boom and receptacle damage, and receiver airplane over-runs with the boom at the inner upper corner of the refueling envelope have resulted in more serious cases of tanker and receiver airplane damage. Maintaining the present known width of the KC-135 boom automatic limits for the refueling disconnect envelope, during all combinations of boom extension and elevation angles, would produce a disconnect envelope with increased safety margins over the current capabilities. The boom gimballing arrangements of the present invention, produces a refueling envelope with increased safety margins. The service history of the tanker airplane refueling operations, indicates that a design disconnect envelope which has an elevation capability of 20.degree. to 40.degree., is acceptable provided that there is sufficient aerodynamic boom control power available for full utilization of these limits.